Conventionally, in a radio communication system such as a mobile communication system, there has been widely used adaptive array control that adaptively controls directivity of radio signals by using a plurality of element antennas, the radio signals being to be transmitted from a radio base station to a communication target radio communication device, for example, a radio communication terminal. With the adaptive array control, directivity of radio signals can be controlled depending on a position of the radio communication terminal.
However, a radio communication system using the adaptive array control has a problem that directivity of radio signals to be transmitted according to a position of a radio communication terminal cannot be controlled appropriately if the radio communication terminal moves during ongoing communications.
Hence, there is proposed a method of estimating Doppler variations (Doppler frequencies) on the basis of radio signals received from the radio communication terminal, and controlling the directivity of the radio signals by using a predetermined number of element antennas by using a predetermined number of element antennas whose estimated Doppler variations are small (for example, Patent Document 1).
With such a method, element antennas with small Doppler variations are selected from among all the element antennas, and therefore, it is believed that excellent directivity of the radio signals can be ensured even when the radio communication terminal moves during ongoing communications.
Patent Document 1: Japanese Patent Application Publication No. 2003-198508 (Page 8, FIG. 5)